Numerous forms of pyrotechnic gas generators, i.e., inflators, are known in the prior art. These inflators, as a class, are typically capable of producing a large volume of gas at a low temperature within a short period of time by the combustion of a solid pyrotechnic gas generant material. The gas thus produced is used, for example, to rapidly expand an inflatable restraining structure, such as an air bag, within a motor vehicle to protect the driver and/or passengers in the event of a collision involving the vehicle.
Inflators presently in use typically include an inner ignition chamber containing means for igniting the solid gas-generating composition and a surrounding combustion chamber wherein the main propellant charge is stored prior to the actuation of the inflator. The combustion of the main charge also takes place within the combustion chamber. The typical inflator further comprises means, located outwardly of the combustion chamber, adapted for cooling and filtering the gaseous combustion products produced by the combustion of the main propellant charge. The cooled and filtered gas then exits from the inflator housing through a plurality of diffuser ports spaced along the periphery thereof and enters and inflates an air bag associated therewith.
The most common means for initiating combustion of the main propellant charge within a pyrotechnic inflator of the type described above is an electrically actuated squib device. Such squibs are typically located within the ignition chamber of the inflator. An example of a squib useful for such an application is set forth in U.S. Pat. No. 3,971,320 to Lee, which discloses a device comprising a nonconductive, i.e., plastic outer case and a metallic inner cup, to minimize the danger of accidental firing by discharge of static electricity or by short circuits.
Typically, the squib is operatively associated with at least one sensing device, of a type well-known in the art, positioned at a remote location upon the body of the vehicle. Sensors of the type commonly in use are adapted to respond to a sudden deceleration in the progress of the vehicle as would normally occur in a collision between the vehicle and another object. Upon a determination that such a collision is imminent, the sensor typically transmits an electrical signal through one or more connecting wire leads to the squib, which results in the combustion of a small pyrotechnic charge located within the body of the squib. The hot gasses and particulates produced upon the ignition of the squib are then directed through ports in the structural members defining the ignition chamber of the inflator, so as to impinge upon the main propellant charge, thus causing it to ignite in turn.
The ignition train described above often additionally includes a packet formed, for example, of aluminum, containing a material such as boron potassium nitrate (BKNO.sub.3) for enhancing the ignition of the main propellant charge. The BKNO.sub.3 material burns with a very hot flame that is suitable for igniting the solid main propellant. This packet is typically positioned within the ignition zone of the inflator device and maintained in position adjacent the initiator means by some type of spring clip arrangement. Examples of such an ignition packet are described in U.S. Pat. Nos. 4,547,342 to Adams et al and 4,722,551 to Adams.
While the boron potassium nitrate is very successful under normal conditions in igniting the main propellant charge, it does suffer from a serious deficiency in that the autoignition temperature of the BKNO.sub.3 is extremely high, i.e., from about 600-700.degree. F. Since the compositions typically chosen for the main propellant charge ignite at an even higher temperature, in the conditions normally encountered, for example, in a car fire, the material within the ignition packet would have to reach a temperature of in excess of 600.degree. F. before it would ignite and thus ignite the main propellant charge.
In this situation, the generator housing would be subjected to even higher temperatures, i.e., in the range of from 800-900.degree. F. Under such conditions, due to laws of temperature and pressure well known to those skilled in the propellant art, the main propellant charge would burn very rapidly and generate gas at an extremely high pressure, thus creating a situation wherein an explosive fragmentation of a heat-weakened generator housing is a distinct possibility. This fragmentation would be even more likely to occur in inflators having housings formed of aluminum, such as those currently being sold by a number of manufacturers in order to permit a reduction in the weight of the inflator unit.
As described in U.S. Pat. No. 4,561,675 to Adams et al., manufacturers have therefore begun to incorporate an additional pyrotechnic composition, i.e., an autoignition material, into the ignition packet in combination with the BKNO.sub.3. This additive must remain stable over an extended period and must autoignite at a temperature lower than the BKNO.sub.3, (about 350.degree. F.). The ignition of this material at 350.degree. F. causes the BKNO.sub.3, and therefore the main propellant charge, to ignite at temperatures significantly lower than the 600-700.degree. F. range disclosed above. As a result, the main propellant charge would burn at a much slower rate and at a significantly lower pressure. Thus, the end effect of adding an autoignition material to the BKNO.sub.3 ignition enhancing composition is to prevent weakening of the inflator housing which otherwise would occur at elevated temperatures (such as those encountered in a car fire), most particularly in aluminum inflators. This, in turn, diminishes the chance of an explosive overpressurization in the event of such a car fire.
The utilization of such prior art ignition material packets is, however, subject to several drawbacks. BKNO.sub.3, for example, is known to be a hygroscopic material. It therefore must be hermetically sealed within the packet to prevent contamination by atmospheric moisture. Such a seal is not always obtainable, however, with the materials and methods utilized in the prior art. As a result, an inflatable occupant restraint device associated with an inflator incorporating such a prior art packet may fail to properly deploy if the ignition train is interrupted or slowed due to contamination of the pyrotechnic materials with water vapor.
Moreover, prior art ignition material packets have either been constructed entirely of plastic or of metal. As discussed above, an all plastic assembly may not be or may not remain hermetically sealed, and thus the material within the packet may be contaminated with moisture to its detriment. Alternately, an all metal assembly requires the provision of an additional barrier between the surrounding metal inflator housing and the ignition material packet in order to prevent a premature initiation of combustion due to the presence of stray electrical charges or short circuits within the inflator unit.